Recently, laminated-type metal gasket material plates comprising a metal plate and a rubber layer, which is provided thereon through an adhesive layer are widely used as a material for metal gaskets, which is placed around an engine. Sealing of combustion gas, lubricant oil and cooling water in the engine is realized by using a gasket that is made by such metal gasket material plates.
However, with such metal gasket material plates, there exists a problem of exfoliation of the rubber layer. Such exfoliation is likely to occur if water, LLC or electrolyte aqueous solution such as salt water, which is supposed to be sealed in the gasket, permeates the surface of the metal plate and exfoliates the coating and to cause a blister between the metal plate and the adhesive layer or between the adhesive layer and the rubber layer.
Therefore, in the conventional method, to solve the above problem, using an impregnation method, a roll coat method or the like, a coating-type chromate treatment agent containing hexavalent chromium compounds, phosphoric acid and silica is coated on the surface of the metal plate as a surface preparation agent. The coated chromate treatment agent is then dried to form a laminated coat. Since such chromate-based surface preparation provides a specific function of hexavalent chromium that (1) is a barrier effect against corrosive factors, (2) is self-repairable, i.e. ability to repair the defective portion in the coating, and (3) has distinguished interface adherence, adhesiveness between the metal material and the adhesive agent is improved.
However, due to recent environmental concerns, use of the chromate treatment, which utilizes hexavalent chromium (chromium (VI)) compounds), for surface preparation of the metal gasket material is avoided in consideration of industrial waste emission and influences of hexavalent chromium on the environment and the human body.
On the contrary, as for metal plates provided by surface preparation without using hexavalent chromium (meaning that the above advantages (1) to (3) are not realized) but with a coating of a normal rubber layer onto the surface through the intermediary of a normal adhesive layer, permeating of water, LLC or electrolyte aqueous solution such as salt water can occur, and as a result there still remains the problem that exfoliation of the rubber layer is likely to occur by exfoliation of the coating or the blister between the metal plate and the adhesive layer or between the adhesive layer and the rubber layer.